1) Field of the Invention
The present invention relates to a sintered body consisting mainly of beta alumina used as a solid electrolyte (hereinafter, abbreviated as beta alumina sintered body) and a method of manufacturing the same and more, particularly relates to a beta alumina sintered body used for a sodium sulfur secondary cell and a method of manufacturing the same.
2) Related Art Statement
Since a beta alumina sintered body has a high sodium ion conductivity, it can be applied to a sodium sulfur secondary cell as a solid electrolyte for use in insulating metallic sodium as a cathode active material and molten sulfur as an anode active material thereof.
In the sodium sulfur secondary cell, almost all the inner resistance thereof is generated due to the solid electrolyte. Therefore, in order to minimize a decrease in output power when the cell is discharged and in order to reduce power loss when the cell is charged, it is desired to minimize the relative resistance of the cell within a preferred temperature range for using the beta alumina sintered body, for instance, at a temperature of 350.degree. C. Further since the life of the sodium sulfur secondary cell depends upon the life of the solid electrolyte, i.e. the life of the beta alumina sintered body, it is desired that the beta alumina sintered body has high strength.
However, the beta alumina sintered body has a drawback in that its optimum sintering temperature range is small. Therefore, when the beta alumina sintered body is sintered at a temperature lower than its optimum sintering temperature range, the sintered body has a low density and many pores are obtained. On the contrary, if the beta alumina is sintered at a temperature higher than its optimum sintering temperature range, extraordinary grains, which make the strength of the sintered body small, are grown in the sintered body, so that the thus obtained sintered body would not be of practical use. Therefore, the beta alumina should be sintered with a great circumspection; and it is difficult to produce the beta alumina sintered body in great quantities in a big furnace, because a temperature distribution is generated in such a big furnace.